An isolated rat kidney preparation has been devised which functions more than two hours during which normal glomerular filtration rates are sustained and more than 98 per cent of filtered sodium is reabsorbed. An entirely synthetic perfusion medium free of red blood cells is used. This allows examination of the effects of the addition of hormones and other physiologically active agents, singly or in combination, on renal circulation and function. A dissection of the factors which control renal circulation and function. A dissection of the factors which control renal circulation and function is thus possible divorced from the multitudinous factors which influence the activity of this organ in vivo. BIBLIOGRAPHIC REFERENCES: Ross, B.D., Epstein, F.H. and Leaf, A. (1975) Metabolic requirement for sodium reabsorption in the perfused rat kidney. In: Biochemical Aspects of Renal Function, Ed. S. S. Angielski and U.C. Dubach (H.Huber, Berne). Weinberg, J., N. Frega and A. Leaf (1976). The role of glucose in sodium transport in the isolated perfused kidney. Clin. Research 24:470A.